1. Field of the Invention
The present invention relates to a semitransparent liquid crystal display element provided with a liquid crystal panel composed of a plurality of pixels each having a reflective portion and a transparent portion arranged therein.
2. Description of the Related Art
Configurations of conventional semitransparent liquid crystal display elements are described in Jpn. Pat. Appln. KOKAI Publication No. 2000-137217 and Jpn. Pat. Appln. KOKAI Publication No. 2003-233069. For a semitransparent liquid crystal display element that is provided with a liquid crystal panel composed of a plurality of pixels each having a reflective portion and a transparent portion arranged therein, the balance between reflection properties and transmission properties are expected to be improved, and better productivity is requested.
The configuration of a semitransparent film that is provided in a semitransparent liquid crystal display element is based on the formation of an ideal circular polarization sheet. In order to form the ideal circular polarization sheet, the optical axis of an ideal λ/4 sheet should only be adjusted so that it is inclined at an angle of 45 degrees to a Pol absorption axis.
Actually, however, there is no ideal λ/4 sheet that has a λ/4 function in the whole wavelength region. Therefore, a method is generally adopted such that an ideal λ/4 sheet is approximated by a combination of an RF equivalent to λ/4 and an RF equivalent to λ/2.
When the ideal λ/4 sheet is thus approximated by combining the RF equivalent to λ/4 and the RF equivalent to λ/2, an important point is how to optimize the respective axial angle layouts and R-values of the RF equivalent to λ/4, RF equivalent to λ/2 and Pol for liquid crystal cells.
In general, the axial angle layout and R-value are optimized for liquid crystal cells by means of a simulation tool, such as an LCD master. However, this method is not efficient, since the axial angle layout and R-value must be optimized every time optical constants, such as liquid crystal materials of the liquid crystal cells and RFs, RF material, twist angle, etc., change.
In order to solve this problem, a method is proposed to settle an axial angle configuration that depends little on changes of the optical constants of the liquid crystal cells and RFs.
First, there will be described a method of settling the angle of the optical axis of a λ/4 sheet with respect to a liquid crystal molecule director of a liquid crystal cell. Usually, the angle of the optical axis is determined with respect to a liquid crystal molecule that adjoins a substrate. If the angle of the optical axis of the λ/4 sheet with respect to the liquid crystal molecule director is optimized, however, the angle of the optical axis of the λ/4 sheet can be optimized without depending on the change of the twist angle of the liquid crystal molecules. The following is a specific description of this optimization method.
Let it first be supposed that the optical axis of the λ/4 sheet that is formed by combining the RF equivalent to λ/4 and the RF equivalent to λ/2 exists along the direction of a synthetic vector of individual optical axes. The optical axis of the RF equivalent to λ/4 is aligned with the direction of the liquid crystal molecule director, and an angle between the RF equivalent to λ/2 and the Pol absorption axis is optimized. If an angle between the λ/4 sheet and the λ/2 sheet is α, and if either an angle between the λ/4 sheet and the Pol or an angle between the λ/2 sheet and the Pol, whichever is smaller, is β, the following relational expression must be established:35≦(α/2)+β≦55.  (Expression 1)Thus, in order to manufacture a circular polarization sheet, an angle between the optical axis of the ideal λ/4 sheet and the Pol must be set to range from 35 degrees to 55 degrees.
The angle between the λ/2 sheet and the Pol is adjusted to meet the condition given by Expression 1.
FIG. 4 is a graph showing the relation between the twist angle and transmittance of liquid crystal molecules in a conventional semitransparent liquid crystal display element. FIG. 4 shows the relation between the twist angle and transmittance of the axial angle configuration that is settled by the aforementioned method. The axis of abscissa represents the twist angle, while the axis of ordinate represents the transmittance standardized at 62 degrees. Based on an angle x between the transmission axis of the polarization sheet and the polarization direction of transmitted light that is delivered from the liquid crystal cell and transmitted through the RF equivalent to λ/4 and the RF equivalent to λ/2 in the order named, the transmittance is obtained according to Expression 2 as follows:Transmittance=cos2(x).  (Expression 2)
If the polarization direction of the aforesaid transmitted light is aligned with the transmission axis of the polarization sheet, therefore, the transmittance is 100 percent (%).
As shown in FIG. 4, the transmittance is 2 when the twist angle of the liquid crystal molecules is zero degree. When the twist angle of the liquid crystal molecules is 45 degrees, the transmittance is 1.4. If the transmittance is 1 when the twist angle of the liquid crystal molecules is 62 degrees, the transmittance is 1.4 when the twist angle of the liquid crystal molecules is 45, and the transmittance is 2 when the twist angle of the liquid crystal molecules is 0 degree. Thus, the greater the twist angle of the liquid crystal molecules, the lower the transmittance is.
The following is a description of the balance between optical properties and manufacturing margins that are indicative of the ease of manufacture of semitransparent liquid crystal display elements. FIG. 5 is a diagram showing the relation between the twist angle and a gap fluctuation range Δd of the liquid crystal molecules in the conventional semitransparent liquid crystal display element.
FIG. 5 shows the result of examination of the gap fluctuation range Δd that is allowed by an optical property for reflection with respect to each twist angle. When the twist angle of the liquid crystal molecules is zero degree, the gap fluctuation range Δd is ±0.1 micrometer (μm). When the twist angle of the liquid crystal molecules is 45 degrees, the gap fluctuation range Δd is ±0.4 micrometer (μm). When the twist angle of the liquid crystal molecules is 62 degrees, the gap fluctuation range Δd is ±0.8 micrometer (μm).
As seen from the results shown in FIGS. 4 and 5, the transmittance is at its highest, 2.0, if the twist angle of the liquid crystal molecules is zero degree. Since the gap fluctuation range Δd that indicates a margin for reflection properties is as narrow as ±0.1 micrometer (μm), however, it is very hard to manufacture a semitransparent liquid crystal display element. This is because the gap fluctuation range Δd for normal manufacturing processes for semitransparent liquid crystal display elements should be as wide as about ±0.3 micrometer (μm).
Thus, the gap fluctuation range Δd for normal manufacturing processes for semitransparent liquid crystal display elements is expected to be about ±0.3 micrometer (μm). Preferably, therefore, the twist angle of the liquid crystal molecules should be 45 degrees with which the gap fluctuation range Δd is ±0.4 micrometer (μm), which is approximate to about ±0.3 micrometer (μm).
If the twist angle of the liquid crystal molecules is adjusted to 45 degrees, in view of the manufacturing margins that are indicative of the ease of manufacture of semitransparent liquid crystal display elements, as mentioned before, however, the transmittance is much lower as compared to the case where the twist angle of the liquid crystal molecules is zero degree.
The object of the present invention is to provide a semitransparent liquid crystal display element free from lowering of transmittance and easy to manufacture.